1. Field of the Invention
The present invention relates to an apparatus and a method for detecting a road friction coefficient and more particularly to an apparatus for detecting a proper road friction coefficient according to road conditions.
2. Description of the Prior Art
In recent years, numerous vehicle control technologies such as a traction control technology, a braking force control technology, a torque distribution control technology and the like, have been proposed and some of them have been realized in actual automobiles.
In many of these vehicle control technologies, road friction coefficients are essential control parameters and it is a key technology to find an accurate road friction coefficient. In Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-2274, the applicant of the present invention proposes a technique in which an estimated road friction coefficient is calculated based on a steering angle .delta., vehicle speed V, a yaw rate .gamma. and other parameters in accordance with a so-called adaptive control theory.
According to the technique disclosed in the patent No. Toku-Kai-Hei 8-2774, a road friction coefficient can be estimated by estimating a tire characteristic on the basis of time based on the comparison of a yaw and lateral motion model with a yaw or lateral motion of the actual vehicle.
However, in this estimation method of the road friction coefficient, since the calculation is executed through integration based on an estimated initial value, if the estimated initial value is inproper, it takes a long time to reach a final estimated value and this adversely affects the responsibility of the road friction coefficient detecting apparatus. Further, this estimation method has a problem of that it is difficult to estimate a proper road friction coefficient due to the uneveness of road surfaces, when the vehicle travels on rough roads, because this estimation method needs repeated inputs of yaw responses to steering operations.